1. Field of the Invention
This invention relates to molding compositions for ethylene polymers and more particularly to such a molding composition comprising ultrahigh molecular weight polyethylene obtainable by employing a selected mode of multi-stage polymerization reaction and a catalyst of a selected composition therefor.
2. Description of the Prior Art
An ethylene polymer with an extremely high molecular weight of about one million or greater is generally known as ultrahigh molecular weight polyethylene (hereinafter referred to simply as "UHMW polyethylene"). UHMW polyethylene is known in common use as an engineering plastic material characterized by high impact and wear resistance and self-lubrication, and hence it has found extensive application to hoppers, silos, gears, linings and the like for use in the various fields of industry including food processing, civil engineering, chemistry, agriculture and mining, and further to backings for skis, and sports and leisure supplies.
Because of its high molecular weight, UHMW polyethylene is literally too viscous for molding in hot melt and even in solution; that is, it is only marginally moldable. This presents a bottleneck in the efforts to open up new areas of market for UHMW polyethylene in spite of the aforesaid excellent properties.
In order to cope with this problem, there have been proposed certain improved modes of UHMW polyethylene molding, typical examples of which are an injection molding method as disclosed in Japanese Patent Application (Kokai) No. 57-82031 and an injection-compression molding method as disclosed in Japanese Patent Application (Kokai) No. 57-169335. Such known molding methods however involve preforming UHMW polyethylene in a particulate form into pellets or similar shapes.
Alternatively, polyolefin waxes and petroleum resins have been used as additives to improve the moldability of UHMW polyethylene, but their compatibility was not satisfactory, resulting in molded articles of reduced mechanical strength and low impact resistance.